Impact of work teams: a comparison study of assembly cells and assembly line for a variety of operating environments

This research compares assembly lines with different cellular, team working environments. Organization of an assembly system using cells requires worker flexibility where each worker is allowed to help his or her co-workers in the same cell and often to undertake a number of different tasks within the cell. We compare two different cellular systems with an unpaced, serial assembly line. The manufacturing systems are initially compared through an exploratory Markovian approach. Subsequently, the relative performance levels of the systems are examined across a variety of operating environments using simulation models. Operating environments vary based on shop size, setup times and processing time variances. The results indicate the general superiority of the cellular systems over the assembly line. However, some interesting observations are drawn for cases where the assembly line outperforms the cells. One of the cases is where workers realize substantial benefits in terms of performing their tasks in less time due to specialization on the tasks. The other scenario is where working in teams make the overall completion of the tasks more inefficient compared to working individually. Specific cross-over points are analyzed where the assembly lines start performing better than the assembly cells under these scenarios. The conclusions highlight guidelines for practicing managers on the most appropriate system under a given set of operating conditions.     

On the performance of hybrid multi-cell flexible manufacturing systems

Industrial experience has shown that it is virtually impossible to implement a large-scale flexible manufacturing system (FMS) without using the group technology manufacturing concept. However, grouping machines into product cells can limit the FMS flexibility. Thus when the production cells are not completely disjoint, problems under multi-cell flexible manufacturing systems (MCFMS) can be caused by changes in job mix and demand which lead to a workload imbalance both between cells and between machine centres within the same cell. The problems can be mitigated and shop performance improved by transferring workloads from a congested machine centre in one cell to an alternative, less congested machine centre in another cell. Such inter-cell workload transfer results in a hybrid MCFMS which is a cross between a parts similarity-based MCFMS and a process similarity-based MCFMS. Results of a simulation study carried out by the author show that inter-cell workload transfer is very effective in improving shop performance. This paper briefly describes the simulation study and discusses the implications of its results for the design and operation of FMSs. The operational viability, and economic feasibility of hybrid MCFMSs are also discussed in the paper.     

The application of workload control in assembly job shops: an assessment by simulation

Workload control (WLC) is a production planning and control concept developed to meet the needs of small- and medium-sized make-to-order companies, where a job shop configuration is common. Although simulation has shown WLC can improve job shop performance, field researchers have encountered significant implementation challenges. One of the most notable challenges is the presence of ‘assembly job shops’ where product structures are more complex than typically modelled in simulation and where the final product consists of several sub-assemblies (or work orders) which have to be co-ordinated. WLC theory has not been developed sufficiently to handle such contexts, and the available literature on assembly job shops is limited. In response, this paper extends the applicability of WLC to assembly job shops by determining the best combination of: (i) WLC due date (DD) setting policy, (ii) release method and (iii) policy for coordinating the progress of work orders. When DDs are predominantly set by the company, the DD setting policy should play the leading role while the role of order release should be limited and the progress of work orders should not be co-ordinated in accordance with the DD of the final product. But when DDs are predominantly specified by customers, the importance of order release as a second workload balancing mechanism increases and work orders should be coordinated by backward scheduling from the DD of the final product. Results indicate that WLC can improve performance in assembly job shops and outperform alternative control policies. Future research should implement these findings in practice.     

Process plan selection based on product mix and production volume

Most shops currently maintain a single process plan for each part type manufactured by the shop even when multiple process plans are feasible to produce the part. The use of a static process plan for a part regardless of the product mix and volume robs the shop of production flexibility and efficiency. In this paper, given that multiple process plans for a part exist, a method to determine the best process plan to implement for a part type in a given production scenario defined by a known product mix and volume is addressed. The method selects a set of process plans to implement based on minimizing total material handling and machining time for the part mix and volume. The problem is modelled mathematically and solved using a heuristic algorithm. Experimental results to describe the performance of the algorithm are presented for different production scenarios, problem sizes, and solution strategies.     

Layout design in fractal organizations

Today's complex, unpredictable and unstable marketplace requires flexible manufacturing systems capable of cost-effective high variety–low volume production in frequently changing product demand and mix. In fractal organizations, system flexibility and responsiveness are achieved by allocating all manufacturing resources into multifunctional cells that are capable of processing a wide variety of products. In this paper, various fractal cell configuration methods for different system design objectives and constraints are proposed. These parameters determine the level of interaction between the cells, the distribution of different product types among the cells and the similarity of cell capabilities. A tabu-search-based method is proposed to optimize the product distribution to the cells and the arrangement of machines and cells on the shop floor. This optimization is performed for different fractal cell configuration methods and cell quantities. The quality of the resulting shop floor layouts is measured in terms of resource requirements and material movements. The results indicate that in fractal layouts, a trade-off is required between machine quantities and material travelling distance. It was generally possible to reduce travelling distances by increasing the degree of optimization on machine layout and product distribution for a specific product demand and mix.     

Synchronized manufacturing at final assembly and feeder shops

Many manufacturing environments consist of a final assembly shop that receives components from various feeder shops. Suppose the final assembly shop assembles a mix of products, each requiring varying amounts of different components. Thus, the sequence in which products are assembled imposes time-dependent demands on the feeder shops. Such problems are often found in the assembly of high-tech telecommunications systems and in the assembly of mixed models of automobiles. We develop a variety of models that link the work-in-process inventory in the feeder shop and the finished component inventory to the component production interval and the final assembly sequence. The manufacturing disciplines considered include push, look-ahead and kanban. Feeder shops are assumed to manufacture components continuously, periodically, or at a fixed lot size. Simple performance characteristics of the feeder shop are derived to provide insight into synchronized manufacturing issues and to help evaluate the effect of different final assembly sequences on feeder shops.     

Bi-criteria SDST hybrid flow shop scheduling with learning effect of setup times: water flow-like algorithm approach

In studies on automatic scheduling problems, processing times do not differ according to repetition of job or process sequences so it may also be necessary to consider processing times independent from setup times. While considering setup times, the human factor has an important effect on setup, so by the processing of similar tasks frequently worker skills improve and they are able to perform setup at a greater pace. This fact is known as the ‘learning effect’ in the literature. This paper deals with sequence-dependent setup times (SDSTs) hybrid flow shop scheduling with learning effect of setup times for minimising weighted sum of makespan and total tardiness. A mathematical programming model that incorporates these aspects of the problem is developed which belongs to the NP-hard class. Thus, because of the intensive computation, we propose a novel meta-heuristic approach called water flow-like algorithm (WFA) which has the feature of multiple and dynamic numbers of solution agents. Various parameters of the problem and the WFA are reviewed by means of Taguchi experimental design. For the evaluation of the proposed WFA, problem data was generated to compare it against a random key genetic algorithm (RKGA). The results demonstrate the high performance of the WFA with respect to the RKGA.     

A hybrid artificial bee colony algorithm for flexible job shop scheduling with worker flexibility

The traditional flexible job shop scheduling problem (FJSP) considers machine flexibility but not worker flexibility. Given the influence and potential of human factors in improving production efficiency and decreasing the cost in practical production systems, we propose a mathematical model of an extended FJSP with worker flexibility (FJSPW). A hybrid artificial bee colony algorithm (HABCA) is presented to solve the proposed FJSPW. For the HABCA, effective encoding, decoding, crossover and mutation operators are designed, and a new effective local search method is developed to improve the speed and exploitation ability of the algorithm. The Taguchi method of Design of Experiments is used to obtain the best combination of key parameters of the HABCA. Extensive computational experiments carried out to compare the HABCA with some well-performing algorithms from the literature confirm that the proposed HABCA is more effective than these algorithms, especially on large-scale FJSPW instances.     

An interactive tool for designing manufacturing cells for an assembly job-shop

Cellular manufacturing is often implemented to reduce work in progress, materials handling, set-ups and storage space, as well as to improve quality and worker satisfaction. Wemmerlov and Johnson (1997) have pointed out that cellular configurations do not automatically deliver these advantages. An interactive tool is presented to design manufacturing cells for an assembly shop. The method is based on an analysis of operation sequences and durations and it allows the design of hybrid layouts. We show that a cellular configuration is not always desirable and discuss the conditions where this is so.     

Managing worker flexibility and attrition in dual resource constrained job shops

Worker flexibility is an attractive option for enhancing manufacturing performance, since it provides several strategic advantages, and also allows the firm to buffer against uncertainty. In this paper, we investigate issues related to acquiring a flexible work-force in those dual resource constrained (DRC) job-shops that have high learning costs and the presence of worker attrition. Our results show that significant improvement in traditional shop related measures can be attained at even very high attrition rates by incrementally training each worker in one additional department. However, this improvement comes at the expense of productivity losses which shop managers may not be willing to accept. The nature of the tradeoffs that exist in acquiring this incremental worker flexibility are addressed. Two strategies for improving shop performance, based on incrementally training workers or reducing attrition rates, are also recommended for the DRC shop modelled in our study. Conditions under which either of these two strategies should be pursued by the shop managers are also identified.     

Impact of sequence-dependent setup time on job shop scheduling performance

In a production system using multi-purpose and flexible machines, reducing setup time is an important task for better shop performance. Numerous cases were reported about successful reduction of setup times by standardization of setup procedures. However, setup times have not been eliminated and remain an important element of real production problems for production systems such as commercial printing, plastics manufacturing, metal processing, etc. It is especially critical when the setup time is sequence dependent. In this situation, shop performance cannot be effectively improved without the aid of an appropriate scheduling procedure. Review of the past studies shows that there has not been a significant amount of research done on the scheduling procedure for a dynamic job shop with sequence dependent setup times. This paper investigates the job shop scheduling problems that are complicated by sequence-dependent setup times. The study classifies and tests scheduling rules by considering whether setup time and/or due date information is employed. These scheduling rules are evaluated in dynamic scheduling environments defined by due date tightness, setup times and cost structure. A simulation model of a nine machine job shop is used for the experiment. A hypothetical, asymmetric, setup time matrix is applied to the nine machines.     

Virtual cell formation

The concept of a virtual cellular manufacturing system (VCMS), which operates very much like a traditional cellular manufacturing system (CMS), has attracted considerable attention in recent years. Unlike traditional cellular manufacturing systems, virtual cellular manufacturing systems are most suitable in production environments that experience frequent product mix changes. Whereas, in traditional CMS, the shop floor configuration is fixed, in VCMS the shop floor configuration changes in response to changes in product mix over time. Therefore, the life of a given shop floor configuration continues as long as the product mix remains relatively unchanged. Furthermore, whereas in traditional cellular manufacturing systems, a machine cell occupies a contiguous region of the shop floor, the same is not necessarily true with virtual cells. Virtual manufacturing cells are simply logical cells in which the machines belonging to the same cell need not occupy the same contiguous area. Because cell members are logical instead of physical, machines in a cell can be at any location on the floor. In this paper, we present a methodology for designing virtual manufacturing cells.     

Priority scheduling policies for repair shops

The use of repair as an alternative to the replacement of products is a growing trend in many industries, especially those employing expensive assets. Repair shop environments are characterized by a greater degree of uncertainty than traditional job or assembly shop environments, and this introduces unique managerial complications. In this study, scheduling policies are examined in the repair shop environment where no end-item spares are available or where the spares stocking decision is deferred until the minimum obtainable flowtimes for enditems are established. Previous studies of scheduling policies have focused on the case where spares are allowed and have not considered all of the sources of variation in the repair environment. The model developed in this study incorporates these sources of uncertainty, as well as other factors likely to influence repair shop performance. The results show that the variability in repair shops is sufficiently higher than in traditional job or assembly shops to warrant different scheduling policies than those previously reported. The choice of scheduling policy to provide minimum flowtimes and RMS tardiness is operating-environment specific, and clear guidelines are presented for the manager in a repair shop environment.     

Adaptive scheduling and tool flow control in flexible job shops

The recent manufacturing environment is characterized as having diverse products due to mass customization, short production lead-time, and ever-changing customer demand. Today, the need for flexibility, quick responsiveness, and robustness to system uncertainties in production scheduling decisions has dramatically increased. In traditional job shops, tooling is usually assumed as a fixed resource. However, when a tooling resource is shared among different machines, a greater product variety, routing flexibility with a smaller tool inventory can be realized. Such a strategy is usually enabled by an automatic tool changing mechanism and tool delivery system to reduce the time for tooling set-up, hence it allows parts to be processed in small batches. In this paper, a dynamic scheduling problem under flexible tooling resource constraints is studied and presented. An integrated approach is proposed to allow two levels of hierarchical, dynamic decision making for job scheduling and tool flow control in flexible job shops. It decomposes the overall problem into a series of static sub-problems for each scheduling horizon, handles random disruptions by updating job ready time, completion time, and machine status on a rolling horizon basis, and considers the machine availability explicitly in generating schedules. The effectiveness of the proposed dynamic scheduling approach is tested in simulation studies under a flexible job shop environment, where parts have alternative routings. The study results show that the proposed scheduling approach significantly outperforms other dispatching heuristics, including cost over time (COVERT), apparent tardiness cost (ATC), and bottleneck dynamics (BD), on due-date related performance measures. It is also found that the performance difference between the proposed scheduling approach and other heuristics tend to become more significant when the number of machines is increased. The more operation steps a system has, the better the proposed method performs, relative to the other heuristics.     

A practical method for design of hybrid-type production facilities

A comprehensive method for the design of hybrid-type production shops, which comprise both manufacturing cells and individual workcentres, is presented. The method targets the minimization of the material handling effort within the shop and includes four basic steps: (1) identification of candidate manufacturing cells, (2) evaluation and selection of the cells to be implemented, (3) determination of the intra-cell layout, and (4) determination of the shop layout. For the cell formation step the 1CTMM technique has been enhanced to cater for important practical issues. The layout of each significant cell is determined by a simulated annealing (SA)-based algorithm. Once the sizes and shapes of the selected cells are known the shop layout is determined by a similar algorithm. The resulting hybrid shop consists of the selected cells and the remaining machines. The methodology has been implemented in an integrated software system and has been applied to redesign the shop of a large manufacturer of radar antennas.     

Real-world extensions to scheduling algorithms based on lagrangian relaxation

Recently, efficient scheduling algorithms based on Lagrangian relaxation have been proposed for scheduling parallel machine systems and job shops. In this article, we develop real-world extensions to these scheduling methods. In the first part of the paper, we consider the problem of scheduling single operation jobs on parallel identical machines and extend the methodology to handle multiple classes of jobs, taking into account setup times and setup costs. The proposed methodology uses Lagrangian relaxation and simulated annealing in a hybrid framework. In the second part of the paper, we consider a Lagrangian relaxation based method for scheduling job shops and extend it to obtain a scheduling methodology for a real-world flexible manufacturing system with centralized material handling. This research was supported in part by the Office of Naval Research and the Department of Science and Technology grant N00014-93-1017.     

Material requirements planning with flexible bills-of-material

In conventional Material Requirements Planning (MRP), a Bills-of-Materials (BOM) for products is fixed. If time or quantity as stated in the Master Production Schedule (MPS) is not flexible for the final product, flexible BOM may be introduced to compensate for the inflexibility in the MPS. This paper addresses situations where a flexible BOM could be used to deal with unexpected shortages when using MRP to plan for requirements of dependent demand items. The requirements stated in the MPS are met in a timely manner by allowing the substitution of items for one another in the case of a shortage. A linear programming (LP) formulation is provided to help deal, if possible, with a shortage using the flexibility in the BOM. The LP model identifies a ‘mix’ of lower-level items that satisfy both the reality of shortages and ensure as small a deviation from the default BOM as possible. A detailed example in the food production environment is presented to explain how the flexible BOM concept may be applied.     

The performance of Due Date setting rules in assembly and multi-stage job shops: an assessment by simulation

Setting short yet reliable Due Dates (DDs) is an important early production planning and control task. The majority of job-shop research on DD setting assumes simple product structures without assembly operations. However, in practice, product structures are often complex, and multiple final assembly operations may be required. This paper evaluates the performance of DD setting rules in the context of complex product structures, considering two scenarios: two-level assembly job shops, where orders converge on one final assembly operation; and two-level multi-stage job shops, where a series of assembly operations are undertaken. New rules are proposed which are substantially simpler and more suitable for practical use than those in the literature. These rules are only outperformed by a more sophisticated rule from the wider literature, newly introduced into the context of assembly and multi-stage job shops. Which rule to apply in practice depends on whether a manager considers the improvement in performance more important than the loss of simplicity. Future research should investigate how jobs can be planned and controlled effectively when some or all DDs are set externally by customers rather than internally using a DD setting rule.     

Capability-based distributed layout approach for virtual manufacturing cells

It is shown in the literature that in highly volatile manufacturing environments functional job shops and classical cellular manufacturing systems do not perform well. Classical cellular manufacturing systems are very sensitive to changing production requirements due to their limited flexibility. In order to adapt cellular manufacturing systems to volatile manufacturing environments, the virtual cellular manufacturing concept was proposed in the 1980s by the National Bureau of Standards in USA. This concept is similar to group technology where job families are processed in manufacturing cells. The main difference between a virtual cell and the classic cell is in the dynamic nature of the virtual manufacturing cell; whereas the physical location and identity of classic cell is fixed, the virtual cell is not fixed and will vary with changing production requirements. The virtual manufacturing cell concept allows the flexible reconfiguration of shop floors in response to changing requirements. In the literature, the formation and scheduling process of virtual cells are clearly explained and researched in detail. However, the layout issue is not addressed entirely. Virtual cells are generally formed over functionally divided job shops. Forming virtual cells over a functional layout may adversely affect the performance of a virtual cellular manufacturing system. There is a need to search for different layout strategies in order to enhance the performance. The distributed layout approach may be a better alternative for virtual cellular manufacturing applications. In this research paper, a novel capability-based approach is proposed for the design of distributed layouts. A simulated annealing based heuristic algorithm is developed from the distributed layout. The proposed approach is tested with a problem with real data. An example is also shown in order to give an idea about the superiority of a capability-based distributed layout over the functional layouts in forming virtual manufacturing cells.     

应用SIMOGRAMS方法解决装配生产线瓶颈

详细介绍了SIMOGRAMS,一种新的人机操作分析、联合操作分析的新方法.针对某轮胎公司现有生产能力无法满足市场需求的实际问题,首先确定瓶颈车间-成型车间,然后运用SIMOGRAMS工具,从该车间装配生产的主要产品装配流程入手,对整个装配过程进行联合操作分析,提出了初步改善方案并分析改善效果.最后将装配线从固定式布置改变为产品式布置,根据各基本工作单元所包含的工作量将其组合成工作量基本相等的工作包,并将这些工作包分派给每个操作者或工作站,形成基于几种产品的细胞单元,大幅度提高了手工装配线的生产能力和灵活性.